The present invention relates to a power roller bearing for a toroidal-type continuously variable transmission for use, for example, in a power transmission system of a vehicle.
A half-toroidal-type continuously variable transmission, as shown partially in FIGS. 9 and 10, comprises a power roller 13 between an input disk 11 and an output disk 12. The power roller 13 rotates about a displacement shaft 15 which is disposed on a trunnion 14. The trunnion 14 is supported by a pair of trunnion shafts 16 so as to be swung with respect to a support body 17. Between the trunnion 14 and power roller 13, there is interposed a power roller bearing 18.
The power roller bearing 18 comprises an inner ring 20 composed of a portion of the power roller 13, an outer ring 21 disposed to be opposed to the inner ring 20, a plurality of balls 22 respectively interposed between a raceway 21a formed in the outer ring 21 and a raceway 20a formed in the inner ring 20, a ring-shaped retainer 24 for holding the respective balls 22 in a freely rotatable manner, and a thrust bearing 25 interposed between the outer ring 21 and trunnion 14. The respective balls 22 are rotatably stored in their associated pockets 26 formed in the retainer 24. These pockets 26 are disposed at an equal pitch, that is, at equally distant (equi-distant) positions in the peripheral direction of the retainer 24.
When the toroidal-type continuously variable transmission is in operation, as shown in FIG. 9, the power roller bearing 18 and disks 11, 12 are contacted with each other at two contact points C1, C2, thereby providing a so called two-point pressing state. In FIG. 9, θ designates the contact angle. Therefore, the power roller bearing 18 receives the thrust component of a pressing force P at the contact points C1, C2 and, at the same time, it generates the radial-direction component at the mutually 180° opposed position on the circumference of the power roller 13. Thus, the circular-ring-shaped power roller bearing 18 is compressed in the radial direction and, due to this compression force, the inner ring 20 tends to deform into an elliptical shape.
As a result of this, the load distribution on the circumference of the power roller 13 is caused to vary. The balls 22 rotate at high speeds while they are receiving such variable loads and, therefore, the rolling portions of the balls 22 generate a lot of heat. That is, the power roller bearing 18 is used under the severer conditions than an ordinary bearing.
Also, when the toroidal-type continuously variable transmission is in operation, at the traction contact points C1, C2 for transmission of power between the power roller 13 and the respective disks 11, 12, there are generated such tangential-direction forces Ft as shown in FIGS. 10 and 11. A force 2Ft, which is the sum of two forces Ft respectively generated at the two contact points C1, C2, provides a force Fr (which is shown in FIG. 10) going in a direction to fall down the power roller bearing 18, thereby causing the above-mentioned compression force to unbalance in magnitude.
The orbital speed of the balls 22 of the power roller bearing 18 used under the above conditions provide such distribution as shown by arrow marks in FIG. 12. That is, in case where the rotation direction of the retainer 24 is shown by the arrow mark R, the orbital speeds R1 of the respective balls 22 situated on the 2Ft side are slower than the orbital speeds R2 of the balls 22 situated on the anti-2Ft side.
In this manner, when the power roller 13 rotates, since there are produced orbital speed differences between the respective balls 22, as shown by the line L1 in FIG. 13, the balls 22 are going to roll in such a manner that they are shifted from the above-mentioned their respective equi-distant positions. However, in fact, because the movements of the balls 22 are restricted by the retainer 24, as shown in FIG. 14, the contact loads between the balls 22 and retainer 24 vary according to the positions of the balls 22.
That is, the contact loads P2 of the balls 22 situated on the anti-2Ft side act on the balls 22 so as to push the retainer 24 in the rotation direction R. On the other hand, the contact loads P1 of the balls 22 situated on the 2Ft side act on the balls 22 so as to push the retainer 24 in the opposite direction to the rotation direction R. Due to this, the inner peripheral surfaces of the pockets 26 of the retainer 24 and balls 22 are contacted with each other, which results in the lowered durability of the retainer 24.
Also, when the balls 22 are contacted with the retainer 24, they receive a reactive force from the retainer 24. Due to this, the actual shifting amounts of the balls 22, as shown by the line L2 in FIG. 13, become smaller by a M than their ideal shifting amounts (line L1). That is, the respective balls 22 are caused to slide on the rolling surface by the amount of M, which reduces the efficiency of the toroidal-type continuously variable transmission.
In JP-A-2001-4003, there is disclosed a technique in which, in order to reduce phase differences to be generated between balls, pockets are made slightly longer in the peripheral direction of a retainer (that is, the pockets are respectively formed as elongated pockets) to thereby widen a clearance between the balls and the inner peripheral surfaces of the pockets. According to this conventional technique, in a high load area, the balls are able to shift in the longitudinal direction of the pockets and, therefore, the orbital speed differences of the balls can be absorbed. However, in a low load area, since the phase differences between the balls are small, the balls tend to stay in the vicinity of the centers of the pockets; and, because the above-mentioned clearance is relatively large, there is a possibility that the retainer can be vibrated in the peripheral direction thereof.